By comparing the organization, specificity and function of regulatory elements in Salmonella phage P22 and coliphage lambda, an insight has been gained into both the regulation and evolution of temperate bacterial viruses. We have undertaken a physical and functional analysis of Salmonella phage L to determine its relationship to phages P22 and lambda. We have constructed lambda immL hybrids between phages L and lambda. Both genetic tests and DNA heteroduplex analysis have been used to compare the analogous phage L and lambda genes for recombination, regulation, repression and replication. Except for their similar genetic organization and several small regions of similar DNA sequences, the genomes of these phages are quite different. A variety of well-characterized lambda imm22 hybrids constructed between phages P22 and lambda is presently available. We propose to construct a physical map of the recombination, regulation, repression and replication genes of P22 and L using these hybrid phages. To accomplish this we will also create new combinations of P22 and L genes in lambda-P22-L hybrids. The viability of these tri-hybrids and their genetic properties will supply additional insight into viral evolution. The functional analysis of lambda imm22, lambda immL and lambda-P22-L hybrids will rely on determinations of specificity (can one gene product substitute for another) and possible gene interactions with the host.